Information communication networks typified by the Internet are spread all over the world as the infrastructure indispensable to people's lives. As a technique that supports the traffic of this Internet, there is an optical communication technology using optical fibers. Optical communication devices using a silicon platform capable of using 1.3 μm band and 1.5 μm band among the optical fiber communication wavelength bands have been expected as communication devices capable of realizing high-density optical integrated circuits that can be manufactured at a low price by using the CMOS manufacturing technique.
However, it is believed that it is difficult to couple light propagating through an optical fiber or thorough space with an optical waveguide device using a silicon platform. This is because since the refractivity difference between the core and the clad of a waveguide within an ordinary optical waveguide device is conventionally large, its size is in the order of 1 μm square. Therefore, it is significantly smaller than the spot size of light emitted from an ordinary single-mode optical fiber, which is about 9 μm. Accordingly, it is extremely important in silicon photonics to realize a mode converter having a sufficiently small device size, capable of optically coupling an optical waveguide device with a single-mode optical fiber.
Mode converters that have been proposed in the past can be generally divided into two types, i.e., devices (1) having a taper structure for adiabatically changing the mode cross-sectional area of light and devices (2) having an inverse taper structure for adiabatically changing the mode cross-sectional area of light and having a clad for confining the light attached over this inverse taper structure.
In the case of the latter devices (2), it is necessary to increase the taper length of the inverse taper section and the length of the upper clad section to several hundred μm or longer, and thus it is very difficult to reduce the devices in size.
As for the former devices (1), a structure in which a horizontal taper section and a vertical taper section, which are in such a relation that their central axis directions are perpendicular to each other, are three-dimensionally combined has been proposed. However, its manufacturing process is complicated, and thus making the manufacturing very difficult.
As for the former devices (1), a structure in which a plurality of waveguides having taper sections having different planar shapes are placed on top of one another into a layer structure and the manufacturing process is thereby simplified has been proposed (Patent literatures 1 to 5).
Patent literature 1 discloses in FIG. 4 an optical waveguide structure having a two-layer structure composed of: a first layer (102) having a wide-width section having a relatively wide and uniform width and a narrow-width section having a relatively narrow and uniform width, and having such a shape that these sections are connected with a taper section interposed therebetween; and a second layer (103) formed above the wide-width section of the first layer (102) with a shape smaller than that wide-width section, composed of a wide-width section having the same width as the wide-width section of the first layer (102), and a wedge-shaped section connected to the wide-width section.
Patent literature 2 discloses in FIGS. 1 and 6 an optical waveguide structure having a two-layer structure composed of: a first layer (20, 40) having a wide-width section (21) having a relatively wide width and a narrow-width section (40) having a relatively narrow width, and having such a shape that these sections are connected with a taper section (22) interposed therebetween; and a second layer (30) formed above the wide-width section and the taper section of the first layer (20, 40) with a shape smaller than these sections, composed of a first taper section (31) whose width becomes continuously narrower from one end toward the other end, a connection section (32) which is consecutively connected to the other end of the first taper section (31) and has a uniform width, and a second taper section (33) which is consecutively connected to the connection section (32) and whose width becomes continuously narrower from one end located on the connection section (32) side toward the other end.
Patent literature 3 discloses in FIG. 1 an optical waveguide structure having a two-layer structure composed of: a first layer (4) composed of a taper section whose width becomes continuously narrower from one end toward the other end, and a narrow-width section which is consecutively connected to the other end of the taper section and has a uniform width; and a second layer (5) composed of a wedge-shaped section formed above the taper section of the first layer (4) with a shape smaller than that taper section.
Patent literature 4 discloses in FIG. 1(b) an optical waveguide structure having a two-layer structure composed of: a first layer composed of a wide-width section (lower section of 101) having a uniform width, and a taper section (106) which is consecutively connected to the wide-width section and whose width becomes continuously narrower from one end toward the other end; and a second layer composed of a wide-width section (upper section of 101) formed above the wide-width section of the first layer with the same shape of that wide-width section, and a wedge-shaped section (105) which is consecutively connected to the wide-width section. In Patent literature 4, the wide-width section of the first layer and the wide-width section of the second layer are integrally formed.
Patent literature 5 discloses in FIG. 1 an optical waveguide structure having a two-layer structure composed of: a first layer composed of a taper section (110) whose width becomes continuously narrower from one end toward the other end, and a narrow-width section (116) which is consecutively connected to the other end of the taper section (110) and has a uniform width; and a second layer composed of a wedge-shaped section (112) formed above the taper section (110) of the first layer with a shape smaller than that taper section.
Patent literature 5 discloses in FIG. 3 an optical waveguide structure having a three-layer structure composed of: a first layer composed of a taper section (310) whose width becomes continuously narrower from one end toward the other end, and a narrow-width section (316) which is consecutively connected to the other end of the taper section (310) and has a uniform width; a second layer composed of a wedge-shaped section (312) formed above the taper section (310) of the first layer with a shape smaller than that taper section; and a third layer composed of a wedge-shaped section (313) formed above the second layer with a shape smaller than that second layer.